1. Field of the Invention
The present invention relates to a high-frequency band amplifier system using a surface acoustic wave filter (hereinafter referred to as "the SAW filter").
2. Description of the Related Art
When the impedance of the SAW filter is matched in relation to the impedances of a signal source and a load (hereinafter referred to as "impedance matching") in a high-frequency circuit incorporating the SAW filter, the amount of ripple in the band passed by the filter increases and fails to be put into a practical use. In order to avoid such an inconvenience, the circuit is generally constructed by mismatching the impedance of the SAW filter in relation to the impedances of a signal source and a load (hereinafter referred to as "impedance mismatching").
FIG. 4 shows a comparison of transmission characteristics within the band passed by the filter between the cases where the impedance matching is performed and the impedance mismatching is effected. FIG. 4(A) illustrates a diagram when the impedances of the signal source and the load are respectively matched in relation to the input and output impedances of the SAW filter, while FIG. 4(B) illustrates a diagram when the impedances of the signal source and the load are respectively mismatched in relation to the input and output impedances of the SAW filter. The impedance mismatching conditions are set as follows. The signal source impedance is set as one fourth of the input impedance at the input terminal of the SAW filter, while the load impedance is set as one fourth of the output impedance at the output terminal of the SAW filter. As seen in FIG. 4(A), the amount of ripple in the band is approximately 1.2 dB when the impedance matching is performed, while FIG. 4(B) shows that the ripple can be reduced to approximately 0.4 dB when the impedance mismatching is effected. In general, it is desired that the impedances of the signal source and the load are respectively several times smaller than the input and output impedances of the SAW filter.
When a high-frequency band transistor amplifier system is constructed using a filter, it is conventionally believed to be advantageous to use a common-emitter amplifier to improve the systems characteristics, such as the NF, the gain and the stability, and the like. This is also applied to a system in which a SAW filter is used.
FIG. 5 illustrates a conventional high-frequency band transistor amplifier system using the SAW filter. FIG. 5 shows a signal source 1, a SAW filter 2 and a common-emitter amplifier 3. As shown, the input impedance and the output impedance of the SAW filter 2 can be indicated by an equivalent circuit including parallel circuits R.sub.I, C.sub.I and R.sub.O, C.sub.O, each containing an equivalent resistor and an equivalent capacitor. L.sub.I and L.sub.O are resonance inductances which allow, the SAW filter 2 to produce resonance with the equivalent capacitances C.sub.I and C.sub.O. In view of this background, R.sub.I and the signal source resistor R.sub.S are allowed to be in a mismatched state, and R.sub.O and the load are also allowed to be in a mismatched state. Since the input impedance of the common-emitter amplifier 3, which is used as a load, is relatively high (approximately 12 k.OMEGA.), a resistor R.sub.B used for impedance mismatching and having a small value of approximately 50.OMEGA., is inserted between the output terminal of the SAW filter 2 and the transistor amplifier. Since the high-frequency band circuit is constructed as described above, the load impedance in relation to the output impedance (R.sub.O =200.OMEGA.) of the SAW filter 2 is approximately 50.OMEGA. (the input impedance of the transistor amplifier is negligible), thus reducing the amount of ripple. The signal source resistance R.sub.S in relation to the input impedance (R.sub.I =300.OMEGA.) at the input terminal of the SAW filter 2 is set as 75.OMEGA., thereby realizing the impedance mismatching therebetween.
The noise factor (NF) of an amplifier will now be considered.
It has been theoretically substantiated that the NF of an amplifier is generally determined by the signal source impedance, the input impedance of the amplifier and the equivalent noise resistance peculiar to an amplifier element constituting the amplifier. The input impedance and the equivalent noise resistance are determined by the type of grounded transistor used in the amplifier, and consequently, the NF is determined by the signal source impedance on the whole. The NF of the common-emitter amplifier can be indicated by the curve A shown in FIG. 6, while the NF of the common-base connected transistor amplifier can be indicated by the curve B. The horizontal axis of the diagram shown in FIG. 6 indicates the signal source impedance. As is seen from the curve A, the NF of the common-emitter amplifier obtains a minimum value (approximately 1 dB) when the impedance viewed from the input terminal of the amplifier to the signal source is from 2 to 3 k.OMEGA.. Such impedance conditions are referred to as "NF matching". Such a minimum value is much smaller than that obtained by performing NF matching using the common-base amplifier (approximately 5.3 dB when the signal source impedance is in a range from 100 to 200.OMEGA., on the curve B).
When the SAW filter is used, a resistor R.sub.B used for the impedance mismatching, having a small value (50.OMEGA.), is inserted between the output terminal of the SAW filter and the transistor amplifier in order to reduce the amount of ripple as described above. Under such circuit conditions, the impedance viewed from the input terminal (base) of the transistor amplifier to the signal source is approximately 40.OMEGA., as shown in FIG. 7 (the parallel impedance consisting of the output impedance 200.OMEGA. of the SAW filter and the small resistance R.sub.B of 50.OMEGA.). As a result, the actual NF of the common-emitter amplifier 3 is positioned near the point a (from 8 to 9 dB) on the curve A shown in FIG. 6, and it is understood that such a value significantly deviates from NF matching. That is, in the common-emitter amplifier using the SAW filter, NF matching cannot be achieved if an improvement is made to reduce the amount of ripple in the band passed by the filter, thus ending in a significant increase in the NF.
Conventionally, the impedance ratio of R.sub.I at the input terminal of the SAW filter 2 to the signal source resistance R.sub.S is set to be substantially equivalent to that of R.sub.O at the output terminal of the SAW filter 2 to the small resistance R.sub.B, and accordingly, the mismatching loss produced at the input terminal of the SAW filter is substantially equivalent to that at the output terminal thereof. In general, the presence of a mismatching loss increases the NF. The SAW filter is known to have a relatively large insertion loss (approximately 13 dB), and consequently, an amplifier used as a load is connected to the SAW filter in order to compensate for such a loss. Thus, in the conventional transistor amplifier system, R.sub.S is approximately 75.OMEGA. and R.sub.I is 300.OMEGA., thereby causing a loss of approximately 2 dB, which directly gives rise to an increase in the NF. At the output terminal of the SAW filter, the NF is determined by the relationship between the amplifier and the signal source impedance, that is, the output impedance of the SAW filter, in relation to the amplifier, as shown in FIG. 6. As stated above, the mismatching loss produced at the output terminal of the SAW filter does not directly influence the NF.
FIG. 8 illustrates another example of a conventional amplifier system. In this example, a transistor amplifier 4 is balanced, and the other constructions are similar to those shown in FIG. 5. FIG. 8 also illustrates a resistor R.sub.B used for the impedance mismatching, a constant current source 5 and biasing power sources 6 and 7.
As has been discussed above, conventionally, a common-emitter amplifier is connected to the output terminal of the SAW filter. Hence, although such an amplifier is able to minimize the amount of ripple in the band passed by the filter, it is unable to achieve NF matching, thus ending in a considerable increase in the NF. Further, a loss caused by the impedance mismatching at the input terminal of the SAW filter is set to be substantially equivalent to that at the output terminal thereof, which also results in an increase in the NF. This produces a problem in that a transmitter/receiver for transmitting and receiving faint signals as well as normal signals has a narrow dynamic range.